1. Field of the Invention:
The invention relates to a method of manufacturing plastic, electrically conductive pressure-formed plates from thermoplastic plastic particles which are mechanically formed and are themselves electrically insulating. These particles are then "doped" with particulate, conducting solids.
2. Discussion of the Background:
Ordinarily, the term "plastic material" is associated with the property of high resistance to passage of electric current, i.e., insulation. Conductivities of such materials are in the range G =10.sup.-10 to 10.sup.-18 (ohm-cm).sup.-1. There has been substantial industrial interest in the insulating properties of plastics.
However, occasions arise when it is desirable for polymers to be electrically conducting. Polymers which have regularly alternating double and single bonds as the chief feature of their bonding systems can achieve sigma-values in the metal or semiconductor range, when electron acceptors or electron donors are incorporated in them. (See Weddigen, G., Physik in unserer Zeit, 14, 4:98 (1983); and "Kirk-Othmer", 3rd Ed., published by John Wiley, Vol. 18, pp. 755-93 (1982)). Such polymers include, for example, polyacetylene, polypyrrole, and polysulfur nitride.
Addition of "conducting fillers" can result in increases in the conductivity of polymers which are inherently insulators, such that technically useful conductivities are achieved. Candidates for such fillers include carbon black, lead, and silver. According to Weddigen, noted above, one can reproducibly achieve only a narrow conductivity range, i.e., between 10.sup.-4 and 10 (ohm-cm).sup.-1. The content of the conducting filler is generally 10 to 40 wt.%. At relatively low filler concentrations (about 5 wt.%) the conducting particles do not statistically form conduction paths within the insulator, so that they do not result in overall conductivity. When the filler concentration is increased, there is an abrupt incidence of statistically frequent contacts between conducting filler particles, and consequently an abrupt increase in conductivity to a level close to that of the filler material itself.
At high field strengths, the lines of current flow first run along the paths formed by the filler particles which are in contact with each other. At lower field strengths, filler particles which are close but not touching do not contribute to the current through the piece. At high field strengths there is dielectric breakdown. Such a conductor no longer obeys Ohm's law.
Another disadvantage of the "filler method" is that when the filler is incorporated, the conducting filler particles are not uniformly distributed, due to their density being different from that of the insulating polymer matrix. They are more concentrated in the lower regions. In this regard, "Kirk-Othmer" (loc. cit., p. 767) states: "Doped polymers exhibit a host of additional difficulties associatd with the disorder and gross inhomogeneity of the dopants. Thus, achievement of the goal of making synthetic metals from conducting polymers faces hurdles that were unanticipated as little as a decade ago." Homogeneity of the distribution of the conductivity carriers is therefore an essential prerequisite for industrial use of polymeric conductors.
In view of the importance of a homogeneous distribution of the conducting particles, research was undertaken with acrylates as the matrix to determine the maximal homogeneous distribution of particles in the polymer matrix. This research led to the result that significant conductivity appeared with conducting carbon black only at very high concentrations of the carbon black (&gt;15-25 wt.%). The addition of such substantial amounts of carbon black in polymethyl methacrylate is in practice virtually impossible, due to thixotropy, and results in highly friable materials. It is expected that the same basic situation will apply with other polymer substrates.
In European Patent 0 013 753, a method of preparing electrically conducting polyolefin molded pieces is described in which the conducting carbon black is applied to the surface of polyolefin particles by fusing the carbon black to the surface. Apparently the technical result of the method depends closely on the crystallizability of the polyolefin. Noncrystallizing plastics are thus not candidates for the process.